(1) Field of the Invention
The present invention relates to a direct current motor having E-shaped interpoles, more particularly to a low noise direct current electric motor having E-shaped interpoles used, for example, in a machine tool.
(2) Description of the Prior Art
In order to reduce the magnetomotive force produced by the interpoles and to improve various operating characteristics of the DC motor, a DC motor having E-shaped interpoles has been proposed in, for example, the Japanese Patent Application Laid Open No. 53-126107 corresponding to U.S. patent application Ser. No. 884,586, now U.S. Pat. No. 4,220,882, by the applicant of the present invention.
FIG. 1 illustrates a prior art DC motor having the E-shaped interpoles disclosed in the above-mentioned Patent Application. The DC motor in FIG. 1 comprises an armature 1, a cylindrical shaped yoke 7 and two main magnetic poles 3 and 4 equidistantly spaced around the outer circumference of the armature 1 having a small gap therebetween and attached to the inner circumference of the yoke 7. The main magnetic poles 3 and 4 have field windings 5 and 6 wound thereon respectively. The field windings 5 and 6 are supplied with electric current in a predetermined direction so that the polarities of the main magnetic poles 3 and 4 are selected to be, for example, N and S, respectively. As a result, the armature 1 is counter clockwisely rotated as shown by arrow a. In this case, some of the armature windings 21, 22 and 23, 24, which are located between the main magnetic poles 3 and 4, exist within commutating zones.
In order to eliminate the counter electromotive force induced in the armature windings 21 through 24 existing within commutating zones, the E-shaped interpoles 8 and 9 are attached to the inner circumference of the yoke 7 by using spacers 25 and 26 made of non-magnetic material, and are located at the intermediate positions along the outer circumference of the armature 1 between the main magnetic poles 3 and 4. The interpole 8 comprises a center pole 81 having an interpole winding 10 wound thereon, and two side poles 82 and 83 disposed respectively in front of and to the rear of the center pole 81 along the direction of the rotation of the armature 1. The interpole winding 10 is connected in series with the armature windings 2, and an armature current passes through the interpole winding 10 in such a direction so that the polarity of the center pole 81 becomes S and the polarity of the side poles 82 and 83 becomes N. The other interpole 9 also comprises a center pole 91 having an interpole winding 11 wound thereon, and two side poles 92 and 93. The interpole winding 11 is also connected in series with the armature windings 2, and the direction of the armature current passing through the interpole winding 11 is selected so that the polarity of the center pole 91 becomes N and the polarity of the side poles 92 and 93 becomes S.
The E-shaped interpoles 8 and 9 are hardly affected by the magnetic field produced by the armature reactions. This is because the magnetic flux caused by the whole of the armature current flowing through the armature windings 2 hardly penetrates the interpoles 8 and 9 due to the existence of the spacers 25 and 26 of non-magnetic material. For example, in the interpole 8, only the magnetic flux f.sub.1 and f.sub.2, which are caused by the current passing through the armature windings 21 and 22 existing within a commutating zone, pass through the magnetic circuit including the center pole 81, side poles 82 and 83 of the interpole 8 and the armature 1. Thus, the amount of the magnetomotive force produced by the E-shaped interpoles 8 and 9 can be greatly reduced, and therefore, the cross sectional area of the interpole winding can be very small and the heat generated by the interpoles can be reduced.
In the above-mentioned E-shaped interpole, for example the E-shaped interpole 8, the interpole winding 10 is connected in series with the armature windings 2 in the direction such that the magnetic flux produced by the interpole winding 10 has the opposite polarity to that of the above-mentioned magnetic flux f.sub.1 and f.sub.2, in order to eliminate the magnetic flux f.sub.1 and f.sub.2 and to generate the magnetic flux used for commutation. Consequently, as illustrated in FIG. 2, magnetic flux f.sub.1 ' and f.sub.2 ', whose amount is equal to the difference between the magnetic flux produced by the interpole winding 10 and the magnetic flux f.sub.1 and f.sub.2 produced by the armature windings existing in a commutating zone, pass through magnetic circuits composed of the center pole 81 and the side poles 82 and 83 of the E-shaped interpole 8 and the armature 1. The magnetic flux f.sub.1 ' and f.sub.2 ' cause an attractive force between each of the teeth of the armature 1 and the side poles 82 and 83 and the center pole 81 of the E-shaped interpole 8. Especially, the attractive force between each of the teeth of the armature 1 and the side poles 82 and 83, whose magnetic flux density is relatively high, is very large.
When the armature 1 rotates, each of the teeth of the armature 1 approaches to and departs from the side poles 82 and 83 of the E-shaped interpole 8. Therefore, the side poles 82 and 83 suffer an alternative force whose direction changes alternatively. The repetition frequency of the alternative force changes in proportion to the revolution speed of the armature 1 and the magnitude of the alternative force is in proportion to the magnitude of the armature current. In the condition that the load of the DC motor is heavy or in the condition that the revolution speed of the armature 1 is accelerating or decelerating, the armature current becomes very large so that the alternative force becomes very large. Therefore, in the conventional DC motor having the E-shaped interpoles, the side poles of the E-shaped interpoles vibrate at a relatively high frequency corresponding to the repetition frequency of the alternative force, and thus the conventional DC motor generates a loud noise.